You can upload files to your projects for your team to access, and to attach to other items in your projects, like tasks or comments.
To upload files through the files area of your project, click the Upload button in the top right.
Click the Add files button and browse for files, or drag them onto the window straight from your desktop. (you can choose one or more files to upload at any time):
Once you've added files to this window, if you decide you don't want to upload some of them after all you can deselect the box to the right of the file name. File uploads can also be cancelled during upload, a red x will show to the left of the file name:
You can choose a category, set any notification or privacy options that you prefer, use the description tab to add a description of the file, and in the advanced tab you can choose to enable automatic versioning for the file.
When you're happy with those settings for the file(s) you've chosen, click upload these files. If you click close, the files won't be uploaded to your project.
Note: You can upload up to 2GB of files in a single upload.